TRACTOR DESIGN

By Staff
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461 Algonquin Place, Webster Groves, Missouri 63119

Everyone likes to see the small tractors running around the
grounds at thresher reunions. Whether you have a reproduction of an
Oil Pull or Mogul, or just parts you put together from the scrap
pile, you have a popular display. Kids want a ride and the older
folks want to see how it works,

It doesn’t matter whether you have a fancy machine shop or
just use stove bolts and a 5-pound hammer, some simple rules must
be followed in steering and chassis layout. You don’t repeal
laws of chassis design by ignoring them. If you use these rules,
your tractor will be easy to steer, will drive a straight line over
rough ground and will not scrub the wheels in a turn.

These geometric rules apply to any vehicle controlled by
steering arms and kingpins. Center-pivot tractors like row-crops
and most steam tractors do not have the opportunities for steering
sophistication (or errors) that steering-arm geometry affords. I
have simplified things from what you would need on a 150 miles per
hour race car but everything will work fine at 5 mph. These steps
are easy. Just lay everything out on the floor before you start to
weld.

ACKERMANN STEERING

FIGURE 1: ACKERMANN STEERING

Steering arm layout is very simple but critical. The steering
arms and tie rod can be in front of or behind the axle. Wherever
they are, the Ackermann relationship must be followed. Lines drawn
through the kingpins and the ends of the tie rod and extended must
cross in the center of the rear axle as shown in Figure 1.

Make or bend your steering arms so they fit on the extended
line. Then make the tie rod long enough to fit. Thread one end of
the tie rod to make the length adjustable.

It is almost always easier to place the tie rod behind the axle.
Notice that if the tie rod is in front, it will extend into the
wheels so the spokes will have to be offset to make room. Either
way, the design works and it is 100% a matter of taste and
construction difficulty.

THE DRAGLINK, THE PITTMAN ARM & THE STEERING WHEEL

The placement and size of these three components will determine
how easily the tractor will steer.

Steering ease is a matter of leverage. The levers are the
steering wheel diameter, the Pittman arm length, the steering arm
length and the gears inside the steering box. Figure 2 shows the
layout.

As a rule, the Pittman arm will be longer than the steering arm.
If it is not, you will have a large turning circle and won’t be
able to make sharp turns. Also, use the largest steering wheel you
can to increase leverage.

Length of the draglink is important too. It must be long enough
so that the Pittman arm is vertical to the ground when the front
wheels are straight ahead, as Figure 3 shows.

Incorrect length reduces the leverage but that is outside our
discussion. Just do it right. Also, as the wheels go up and down
over bumps, the geometry is changed but that won’t be important
at the speeds you are going, either.

WHEELBASE AND TRACK

The most important thing about wheelbase and track is to make
sure the tractor will fit on whatever you will use to carry it.
After that, the wheelbase should be at least twice as long as
track. The longer the wheel-base is compared to track the more
forgiving your design will be. Figure 4 shows how to measure
wheelbase and track.

Short wheelbases are cute but there’s not much room for the
parts.

FIGURE 5: CASTER

Caster means that the kingpin leans slightly back at the top.
About five degrees is OK. This gives the tractor stability to
follow a straight course and helps the wheels return to center
after a turn just as a car does. Figure 5 shows caster.

CAMBER

Camber also adds to stability. The more camber you have the more
the tractor will want to go straight, even over rutted ground.
Camber means that the tops of the front wheels are farther apart
than the bottoms. The front wheels of a row crop tractor usually
have extreme camber, for example. The amount is a matter of taste
but do use some. Figure 6 illustrates.

TOE-IN

Toe-in is an adjustment for wear and looseness as far as we are
concerned. When the tractor is rolling, the front wheels should
point just about straight ahead. With all of the looseness in your
front-end, if your wheels point ahead while at rest they will
spread out when you start to drive. To counteract this, set the
fronts of your front wheels about ? inch closer together than the
rears, as Figure 7 shows.

When you start to drive, the slack in the steering is taken up
and the wheels straighten out. If it takes more than ? inch of
toe-in you had better redo that front-end.

If you follow these easy design steps you will have a tractor
that your granddaughter can steer with one hand during the parade
without frightening the spectators. It doesn’t take any more
time to lay everything out correctly, so you might as well do it
right and have fun.

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